Generally, air bearings are used to support a rotating shaft of a high-speed rotating device such as an air cycle machine, a gas turbine, a turbocharger or the like. One of such air bearings is an air foil journal bearing that has a bump foil. The supporting ability of the air foil journal bearing can be determined according to the configuration of the bump foil.
FIG. 1 is a cross-sectional view illustrating an air foil journal bearing and a rotating shaft of the prior art.
As shown in the drawing, an air foil journal bearing 1 comprises: a cylindrical housing 2; a shim foil 4 disposed around the inner periphery of the housing 2; a bump foil 6 mounted on the shim foil 4; and a top foil 8 mounted on the bump foil 6 and supporting a load applied perpendicularly to the axis of a rotating shaft 9. In order to enhance the durability and operational stability at a high temperature, the foils 4, 6 and 8 may be fabricated from an inconel material. In particular, the top foil 8, which contacts the rotating shaft 9 directly, may be coated with a hybrid-solid lubricant or a coating material comprising molybdenum disulfide (MoS2). Also, a coating material 9′ comprising chromium carbide (Cr3C2) may be coated on the rotating shaft 9 to increase anti-abrasion.
However, the above-described coating materials can perform their functions only at a temperature of 500° C. or less. They tend to lose their lubricating ability due to the ignition or thermal deformation at a high temperature of 700˜900° C. Thus, there is a long-felt need to develop a coating material having heat and abrasion resistance and low friction characteristics to support a high-speed rotating shaft in a high temperature (700˜900° C.) and oil-free environment, as well as the need for a coating method that can increase the durability of the coating material.